Bias circuit with voltage and temperature compensation for an emitter coupled logic circuit

ABSTRACT

A bias circuit for providing a reference voltage to an output circuit, for example, an ECL circuit in an LSI. The bias circuit is able to operate at a lower power supply voltage of about -2 V and includes a first transistor having an emitter which is connected to a power supply and a base and a collector commonly connected through an impedance circuit to ground. The bias circuit is also connected to the output circuit, whereby heat generation in the LSI is decreased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a bias circuit. More particularly, it relates to a bias circuit for an emitter coupled logic (ECL) circuit in a large scale integrated circuit (LSI) which can be driven at a relatively low power-supply voltage for the purpose of suppressing heat generation therein.

Generally, in bipolar-type logic circuits such as ECL circuits, the delay time for one logic circuit stage depends largely on the amount of current flowing through the circuit. That is, the delay time in each circuit is inversely proportional to the amount of current flowing therethrough as long as the parasitic capacitances in the circuit, the wiring capacitances of the wires for connecting elements in the circuit, the switching capability of each transistor in the circuit, and the ratio of currents delivered to various elements in the circuit, are constant. In other words, the greater the current supplied to the circuit, the higher the switching speed. Because of this, when trying to decrease the power consumption of integrated circuits, it is better to lower the power supply voltage rather than the current. Reduction of the power consumption of integrated circuits is necessary from the viewpoint of decreasing the amount of heat generated in the circuits, which is one of the factors limiting the degree of integration. The more power consumption in a circuit, the more heat is generated. The power consumption in each gate is especially great in conventional bipolar-type integrated circuits.

Two power supplies are generally used for an ECL integrated circuit, i.e., a power supply for providing a voltage of -5.2 V through -4.5 V to the circuit and a power supply for providing a voltage of -2 V as a terminating voltage at the ends of signal lines. The bias circuit according to the present invention aims to be operated at the voltage of -2 V which is regularly used for the termination of signal lines in ECL circuit systems.

2. Description of the Prior Art

FIG. 1 is a diagram of a conventional bias circuit and a conventional ECL circuit to which a bias voltage is applied. In the figure, the ECL circuit ECL₀ consists of input transistors T₁ and T₂, a reference transistor T₃, a transistor T₄ functioning as a constant current source, an output transistor T₅ for outputting a logical NOR output X₀, an output transistor T₆ for outputting a logical OR output X₀, load resistors R₁ and R₂, and resistors R₃, R₄, and R₅, for current feedback, connected to the emitters of the transistors T₄, T₅, and T₆, respectively. To the base of the reference transistor T₃, a first bias voltage V_(BB0), output from a bias circuit BC₀, is applied. To the base of the transistor T₄, a second bias voltage V_(CS0), output from the bias circuit BC₀, is applied.

The bias circuit BC₀ consists of resistors R₆ and R₇ diodes D₁ and D₂, and a resistor R₈ connected in series between a first power supply terminal V_(CC) and a second power supply terminal V_(EE0). The bias circuit BC₀ also consists of transistor T₇ and T₈ and a resistor R₉ connected in series between the first power supply terminal V_(CC) and the second power supply terminal V_(EE0). The base of the transistor T₇ is connected to a connecting point between the resistors R₆ and R₇. The base of the transistor T₈ is connected to a connecting point between the resistor R₇ and the diode D₁.

The first bias voltage V_(BB0) is determined by the base-emitter voltage V_(BET) of the transistor T₇, which is about 0.8 V, and the voltage drop across the resistor R₆, which is, for example 0.5 V. Therefore, the voltage V_(BB0) is about -1.3 V. The bias voltage V_(BB0) is higher than a low level (L level) of, for example, -1.7 V and lower than a high level (H level) of, for example, -0.9 V.

The operation of the ECL circuit ECL₀ is well known. In brief, when at least one of the inputs A₀ and B₀ is at the H level, the transistor T₁ or T₂ is on, or both transistors T₁ and T₂ are on, and the transistor T₃ is off, resulting, at the NOR output X₀, in the L level of about -1.7 V, which is determined by the voltage drop (0.9 V) across the resistor R₁ and the base-emitter voltage (0.9 V) of the transistor T₅. At the OR output X₀, the H level of about -0.9 V results which is determined by the base-emitter voltage (0.8 V) of the transistor T₆ and the voltage drop (0.1 V) across the resistor R₂. The L level voltage at the output X₀ or X₀ is partially determined by the ratio of the resistors R₁ and R₃. When both of the inputs A₀ and B₀ are at the L level, the transistors T₁ and T₂ are both off, and the transistor T₃ is on, resulting in the H level at the NOR output X₀ and resulting in the L level at the OR output X₀.

Generally, in an ECL circuit, the first power supply voltage (also referred to as V_(CC)) is 0 V, and the second power supply voltage (also referred to as V_(EE1)) is a negative voltage. In order to suppress the generation of heat in the ECL circuit, it is desirable to limit the voltage difference between V_(CC) and V_(EE0) as much as possible. However, when the conventional bias circuit BC₀ is employed, the minimum voltage difference between V_(CC) and V_(EE0) for reliable operation is about 3 V, as explained in the following. That is, the voltage difference between V_(CC) and V_(EE0) is determined by the sum of the voltage drops across the resistor R₆, the base-emitter voltage of the transistor T₇, the base-collector voltage of the transistor T₈, the forward bias voltages across the diodes D₁ and D₂, and the voltage drop across the resistor R₈. The maximum collector-base voltage of the transistor T₈, for keeping its collector-base junction in a reverse bias state in order to prevent the transistor T₈ from being operated in a saturation region, is 0 V. The base-emitter voltage of each transistor and the forward bias voltage of each diode, when manufactured of silicon, are both about 0.8 V. Therefore, the voltage difference between V_(CC) and V_(EE0) should be at least 2.4 V. In order to attain stable operation, the voltage drops across the resistors R₆ and R₈ should be, for example, about 0.2 V and 0.4 V, respectively. As a result, the above-mentioned sum is about 3 V. Therefore, the second power supply V_(EE0) should be lower than -3 V. The absolute value of this voltage is too great for increased integration in an integrated circuit.

In the conventional circuit of FIG. 1, if the second power supply voltage V_(EE0) is -2 V, the base-emitter voltage of the transistor T₇ and the forward bias voltage of the diodes D₁ and D₂ would become insufficient, thereby preventing operation of the bias circuit BC₀.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a bias circuit which can stably operate at a low power supply voltage without deteriorating the switching speed.

Another object of the present invention is to suppress the generation of heat in an integrated circuit.

Still another object of the present invention is to decrease the power consumption in an integrated circuit.

A still further object of the present invention is to increase the degree of integration in an integrated circuit.

In order to attain the above objects, there is provided a bias circuit for providing a bias voltage to an output means comprising: a first transistor having an emitter, a base, and a collector, the base and the collector being connected to each other; a first reference-voltage terminal for providing a first reference voltage connected to the emitter; a substantially constant-voltage node; an output node connected to the base and the collector of the first transistor, for providing the bias voltage; and a first impedance means connected between the constant-voltage node and the output node; the bias voltage being applied to the output means connected to the output node.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and advantages as well as other features of the present invention will be better understood from the following description of preferred embodiments with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of a conventional bias circuit and a conventional ECL circuit to which a bias voltage is applied;

FIG. 2 is a diagram of a bias circuit and an ECL circuit to which a bias voltage is applied, according to a first embodiment of the present invention;

FIG. 3 is a circuit diagram of a second embodiment of the present invention;

FIG. 4 is a block diagram of an example of a system employing conventional ECL circuits; and

FIG. 5 is a block diagram of an ECL large-scale integrated circuit to which the present invention is effectively applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a circuit diagram of a first embodiment of the present invention. In the figure, an ECL circuit ECL₁ is similar to the circuit ECL₀ of FIG. 1 except that the ECL circuit ECL₁ in FIG. 2 does not include the emitter follower output transistors T₅ and T₆, or the resistors R₃, R₄, and R₅. A bias circuit BC₁ in FIG. 2, however, is greatly different from the conventional bias circuit BC₀ in FIG. 1. The bias circuit BC₁ comprises an NPN transistor T₁₀, a PNP transistor T₁₁, NPN transistors T₁₂ and T₁₃, load resistors R₁₀, R₁₁, and R₁₂, and a resistor R₁₃. The resistance value of the load resistor R₁₂ is half that of the load resistor R₁ or R₂. The collector and base of the NPN transistor T₁₀ are commonly connected to a node N. Between the node N and first power supply terminal V_(CC), the load resistor R₁₀ is connected. The emitter of the transistor T₁₀ is connected to a second power supply terminal V_(EE1) to which a power supply voltage of -2 V, which is higher than the conventional power supply voltage of less than -3 V, described later in detail. The PNP transistor T₁₁ has a base connected to the node N, an emitter connected through the load resistor R₁₁ to the first power supply terminal V_(CC), and a collector connected to a third power supply terminal V_(EE2). The voltage at the third power supply terminal V_(EE2) is equal to or less than the voltage at the second power supply terminal V_(EE1). The emitter of the PNP transistor T₁₁ is also connected at a node Nc to one end of the resistor R₁₃. The other end of resistor R₁₃ is connected to an output node O₁ to which the base and the collector of the NPN transistor T₁₂ are commonly connected. The emitter of the transistor T₁₂ is connected to the second power supply terminal V_(EE1). The transistor T₁₃ has a collector connected through the load resistor R₁₂ to the first power supply terminal V_(CC), a base connected to the output node O₁, and an emitter connected to the second power supply terminal V_(EE1). For the circuit configuration of this bias circuit BC₁, the voltage difference between the first power supply voltage, also referred to as V_(CC), and the second power supply voltage, also referred to as V_(EE1), is:

    V.sub.CC -V.sub.EE1 =V(R.sub.11)+V.sub.EBT.sbsb.11 +V.sub.BET.sbsb.10,

where V(R₁₁) is the voltage drop across the resistor R₁₁ ; V_(EBT).sbsb.11 is the emitter-base forward voltage of the PNP transistor T₁₁ ; and V_(BET).sbsb.10 is the base-emitter forward voltage of the NPN transistor T₁₀. V_(EBT).sbsb.11 and V_(BET).sbsb.10 are both about 0.8 V. Since the second power supply voltage V_(EE1) is -2 V and the first power supply voltage is 0 V, the voltage drop V(R₁₁) becomes about 0.4 V. Thus, the voltage difference between the first power supply voltage V_(CC) and the second power supply voltage V_(EE1) can be made as small as 2 V.

Even when the power supply voltage is low, as mentioned above, the ECL circuit ECL₁ can effect stable operation. First, the emitter currents of the transistors T₁₂, T₁₃ and T₄ are same because the base-emitter voltage characteristics of these transistors are the same when these transistors are manufactured during the same manufacturing process. The emitter current of the transistor T₁₂ is equal to the current i(R₁₃) flowing through the resistor R₁₃. The base-emitter voltage V_(BET).sbsb.12 of the transistor T₁₂ is equal to the base-emitter voltage V_(BET).sbsb.10 of the transistor T₁₀. Therefore, the potential at the output node O₁ is equal to the potential at the node N. Accordingly, the voltage V(R₁₃) across the resistor R₁₃ is equal to the emitter-base voltage V_(EBT).sbsb.11 of the PNP transistor T₁₁. Thus, the following expressions can be substantially obtained.

    i(R.sub.13)=V(R.sub.13)/R.sub.13 =V.sub.EBT.sbsb.11 /R.sub.13

Since i(R₁₃) is nearly equal to the emitter current of the transistor T₄ or to the emitter current of the transistor T₁₃, the current flowing through the transistor T₄ or T₁₃ is substantially equal to V_(EBT).sbsb.11 /R₁₃, which is independent from the power supply voltage V_(EE1). In other words, the node N_(c) functions as a substantially constant-voltage node due to the constant voltage characteristics of the forward biased PN junction and enables establishment of a desired constant current i(R₁₃) through the resistor 13. Therefore, even when the power supply voltage fluctuates, the current through the transistor T₄ or T₁₃ is constant.

Second, in operation, when at least one of the inputs A₁ and B₁ is at the H level, i.e., 0 V, the transistor T₁ or T₂, or both transistors T₁ and T₂, are on, so that the above-mentioned current V_(EBT).sbsb.11 /R₁₃ flows through the resistor R₁, resulting in an L level at the NOR output X₁. The L level voltage V_(OLX).sbsb.1 at the NOR output X₁ is:

    V.sub.OLX.sbsb.1 =-R.sub.1 ·V.sub.BET.sbsb.11 /R.sub.13

When the NOR output X₁ is at the L level, the transistor T₃ is off and the OR output X₁ is at an H level of 0 V.

When both inputs A₁ and B₁ are at the L level, the NOR output X₁ is at the H level of 0 V and the OR output X₁ is at the L level of -R₂ V_(BET).sbsb.11 /R₁₃.

The reference voltage V_(BB1) applied to the base of the transistor T₃ is expressed as:

    V.sub.BB1 =- R.sub.12· V.sub.EBT.sbsb.11 /R.sub.13.

As mentioned before, since R₁₂ =1/2R_(1=1/2) R₂, the reference voltage V_(BB1) is: ##EQU1## Thus, the reference voltage V_(BB1) is half of the L level of the NOR output X₁ or the OR output X₁. This means a maximum noise margin for the H level and the L level with respect to the reference voltage V_(BB1).

Third, even when the temperature of the integrated circuit including the ECL circuit ECL₁ is increased, the transistors T₁, T₂, or T₃ are substantially prevented from being saturated. That is, when the temperature is increased, the base-collector voltage of an NPN transistor is generally decreased, so that the transistor tends to enter a saturated state. However, in the circuit of FIG. 2, the base-collector voltage of, for example, the transistor T₁ is -R₁ V_(EBT).sbsb.11 /R₁₃ when it is in an on state. The emitter-base voltage V_(EBT).sbsb.11 of the PNP transistor T₁₁ is also decreased along with the increase of the temperature, because the emitter-base characteristic of the PNP transistor T₁₁ is similar to the base-collector characteristic of the NPN transistor T₁. Therefore, when the temperature is increased, not only the base-emitter voltage of the transistor T₁, but also the base-collector voltage of the transistor T₁ is decreased. Thus, the transistor T₁ is substantially prevented from being saturated even when the temperature is increased. A similar discussion can be applied to the transistors T₂ and T₃.

FIG. 3 is a circuit diagram of a second embodiment of the present invention. The difference between FIGS. 2 and 3 is that, in FIG. 3, resistors R_(S1), R_(S2), and R_(S3), having small resistances, are connected between the emitter of the transistor T₄ and the second power supply terminal V_(EE1), between the emitter of the transistor T₁₃ and the second power supply terminal V_(EE1), and between the emitter of the transistor T₁₂ and the power supply terminal V_(EE1). Except for the above resistors R_(S3), R_(S2), and R_(S1), the circuit configuration of FIG. 3 is the same as that of FIG. 2, and, therefore, a detailed explanation is omitted here. These small resistances function as current feedback devices. More precisely, even when the base-emitter voltages of the transistors T₄, T₁₃, and T₁₂ are made different due to the manufacturing process, the collector currents of these transistors can be made nearly equal to each other by providing the small resistances R_(S1), R_(S2), and R_(S3). That is, these resistors are provided for decreasing current hoggings by the transistors. In a practical circuit design, the small resistances may be provided. It should be noted that the output level does not substantially depend on the small resistances, i.e., the L level of the output X₁ or X₁ is also expressed as -R₁ V_(EBT) /R₁₃. It is preferable to design the L level in the range between -200 mV and -450 mV.

FIG. 4 is a block diagram of an example of a system including ECL integrated circuits. In FIG. 4, reference numerals 1 through 4 are ECL integrated circuits. Each ECL integrated circuit is conventionally driven by a power supply voltage in the range of -5.2 V through -4.5 V. The ECL circuits are connected by transmission lines T to each other. Each of the transmission lines T is connected to an end terminal 5. To each end terminal 5, an end terminal power supply voltage V_(T) is usually applied through a resistor R_(T) having the same impedance as the impedance of each transmission line T. The purpose of applying the end terminal power supply voltage V_(T) through a resistor R_(T) is to prevent signals from reflecting back to the transmission line T. This end terminal power supply voltage is usually -2.0 V. In the embodiments of the present invention described above, this end terminal power supply voltage is used as the second power supply voltage V_(EE1) without providing a particular power supply unit for providing such a low power supply voltage.

FIG. 5 is a block diagram of an ECL large-scale intergrated circuit (LSI) 10. In the figure, the ECL-LSI 10 includes an input gate 11, an internal gate 12, and an output gate 13. All of these gates are connected to the first power supply terminal V_(cc) to which a voltage of 0 V is applied. The input gate 11 and the output gate 13 are driven by the power supply voltage of V_(EE0) in the range between -5.2 V and -4.5 V because, in these gates 11 and 13, the H level is about -0.9 V and the L level is about -1.7 V, and therefore, the logical amplitude is about 800 mV. This value of amplitude is necessary for connecting the ECL LSI 10 to another ECL circuit (not shown). In the internal gate 12, however, the logical amplitude can be made as small as 400 mV by employing the second power supply voltage V_(EE1) of -2.0 V. The interface between the input gate 11 and the internal gate 12 or between the internal gate 12 and the output gate 13 can be easily made. Accordingly, the bias circuit illustrated in FIG. 2 or 3 according to the embodiment of the present invention is effectively included in the internal gate 12, because, as mentioned before, the L level of the ECL circuit connected to the bias circuit of the present invention is preferably selected to be in the range between -200 mV and -450 mV.

From the foregoing description, it will be apparent that, according to the present invention, an improved bias circuit can be obtained which can be stably driven by a low absolute value of a power supply voltage without deteriorating the switching speed, whereby generation of heat in an integrated circuit can be considerably suppressed.

The present invention is not restricted to the above-mentioned embodiments. Various modifications are possible. For example, in place of the ECL circuit, any output means may be provided with the bias voltage produced by the bias circuit of the present invention. 

We claim:
 1. A bias circuit, operatively connected to a power supply, for providing a bias voltage to an output means, comprising:a first transistor of a first conductivity type having an emitter, a base, and a collector operatively connected to the base; a first reference-voltage terminal, operatively connected to the emitter of said first transistor, for providing a first reference-voltage; a substantially constant-voltage node; an output node, operatively connected to the base and collector of said first transistor and to the output means, for providing the bias voltage; first impedance means, operatively connected between said substantially constant-voltage node and said output node, for providing a first impedance, the bias voltage being applied to the output means; second impedance means, operatively connected between said first reference-voltage terminal and said substantially constant-voltage node; a second transistor having a base, having a collector connected to the base and having an emitter operatively connected to said first reference-voltage terminal; a PN junction element including a third transistor of a second conductivity type opposite to the first conductivity type, said third transistor having an emitter operatively connected to said substantially constant-voltage node, having a base operatively connected to the base and collector of said second transistor, and having a collector, the voltage at said substantially constant-voltage node being determined by said PN junction element and said second transistor; and a second reference-voltage terminal operatively connected to the collector of said third transistor.
 2. A bias circuit as set forth in claim 1, wherein said second reference-voltage terminal provides a second reference-voltage equal to the first voltage.
 3. A bias circuit as set forth in claim 1, wherein said second reference-voltage terminal provides a second reference-voltage which is lower than the first reference-voltage.
 4. A bias circuit, operatively connected to a power supply, for providing a bias voltage to an output means, comprising:a first transistor of a first conductivity type, having an emitter, having a base, and having a collector operatively connected to the base; a first reference-voltage terminal, operatively connected to the emitter of said first transistor, for providing a first reference-voltage; a second reference-voltage terminal for providing a second reference-voltage; a substantially constant-voltage node; an output node, operatively connected to the base and collector of said first transistor and to the output means, for providing the bias voltage; first impedance means, operatively connected between said substantially constant-voltage node and said output node, for providing a first impedance means, the bias voltage being applied to the output means; second impedance means operatively connected between said first reference-voltage terminal and said substantially constant-voltage node; a second transistor of a second conductivity type opposite to the first conductivity type, having an emitter operatively connected to said substantially constant-voltage node, having a base, and having a collector operatively connected to said second reference-voltage terminal; and a third transistor having a base and a collector commonly connected to said base of said second transistor, and having an emitter operatively connected to said first reference-voltage terminal, the voltage at said substantially constant-voltage node being determined by said second transistor and said third transistor.
 5. A bias circuit as set forth in claim 4, wherein said second reference-voltage terminal provides a second reference-voltage which is lower than the first reference-voltage.
 6. A bias circuit as set forth in claim 1, wherein said first transistor is of a first conductivity type, and wherein said PN junction element comprises a third transistor of a second conductivity type opposite to the first conductivity type, said third transistor having an emitter operatively connected to said substantially constant-voltage node, having a base operatively connected to the base and collector of said second transistor, and having a collector.
 7. A bias circuit as set forth in claim 6, further comprising a second reference-voltage terminal, wherein the collector of said third transistor is operatively connected to said second reference-voltage terminal.
 8. A bias circuit as set forth in claim 6, further comprising a second reference-voltage terminal, operatively connected to the collector of said third transistor, for providing a second reference-voltage lower than the first reference-voltage. 